Method of and apparatus for producing metal

ABSTRACT

A method involving the production of metal, in particular, aluminum by providing an electrolytic bath containing dissolved oxide of the metal to be produced in a reduction cell. A direct current flows through the bath and metal is collected on the bottom of the reduction cell. The method includes sensing the voltage drop across the cell and determining when this voltage exceeds a given level as an indication of anode effect. The method preferably involves extinguishing the anode effects which may occur. The apparatus for producing metal includes at least one reduction cell having electrodes for delivering direct current to an electrolytic bath containing dissolved oxide of the metal. An anode effect detector is operatively arranged to sense the voltage of the reduction cell and produces an output signal whenever this voltage exceeds a given level indicating the existence of an anode effect. The apparatus preferably includes devices responsive to the output from the anode effect detector for extinguishing the detected anode effects which may occur.

[ June 10, 1975 METHOD OF AND APPARATUS FOR PRODUCING METAL [75] Inventor: Joseph A. Murphy, Murraysville,

[73] Assignee: National-Southwire Aluminum C0.,

Hawesville, Ky.

[22] Filed: Oct. 18, 1972 [21] Appl. No.: 298,405

[52] U.S. Cl. 204/67; 204/225; 204/228; 204/245 [51] Int. Cl. C22d 3/12; B0lk 3/00; C22d 3/02 [58] Field of Search 204/67, 243 R247, 204/225, 228

[56] References Cited UNITED STATES PATENTS 3,434,945 3/1969 Schmitt et a1 204/225 X 3,455,795 7/1969 Boulangcr ct al... 204/225 X 3,539,461 10/1970 Newman et a1 204/67 3,573,179 3/1971 Dirth et al. 204/67 3.578.569 5/1971 Lewis 204/67 X 3,625,842 12/1971 Bristol et a1. 204/245 X 3,627.666 12/1971 Bonfils 204/225 3,674,674 7/1972 Arts et a1. 204/245 X 3,712,857 1/1973 Piller 204/67 3,761,379 9/1973 Elliott 204/225 DECODER Primary Examiner.10hn H. Mack Assistant ExaminerD. R. Valentine Attorney, Agent, or Firm-Van C. Wilks; Herbert M. Hanegan ABSTRACT A method involving the production of metal, in particular, aluminum by providing an electrolytic bath containing dissolved oxide of the metal to be produced in a reduction cell. A direct current flows through the bath and metal is collected on the bottom of the reduction cell. The method includes sensing the voltage drop across the cell and determining when this voltage exceeds a given level as an indication of anode effect. The method preferably involves extinguishing the anode effects which may occur. The apparatus for producing metal includes at least one reduction cell having electrodes for delivering direct current to an electrolytic bath containing dissolved oxide of the metal. An anode effect detector is operatively arranged to sense the voltage of the reduction cell and produces an output signal whenever this voltage exceeds a given level indicating the existence of an anode effect. The apparatus preferably includes devices responsive to the output from the anode effect detector for extinguishing the detected anode effects which may occur.

20 Claims, 1 Drawing Figure SOLENOID DRIVER SOLENOID DRIVER v SOLENOID SOLENOID SOLENOID DRIVER DRIVER 79 DRIVER COMPARATOR CKI COMPARATOR METHOD OF AND APPARATUS FOR PRODUCING METAL BACKGROUND OF THE INVENTION This invention relates to a method of and to an apparatus for the control of an electrolytic reduction cell or cells for producing molten metal. The invention relates, more particularly, to a method of and to an apparatus for the control of an electrolytic reduction cell or cells in which a metallic compound or solute constitutent of a fused electrolyte in an electrolytic cell produces a molten metal. The invention is directed, in its primary adaptation, to the control of an electrolytic cell or cells useful in the production of aluminum.

The production of aluminum by electrolysis of an aluminum containing compound is very old, wellknown process. Commercial aluminum production is carried out by the l-lall-Heroult process in which aluminum oxide, refined from bauxite ore, is reduced electrolytically. Alumina, A1 the solute, is dissolved in molten cryolite, NaF/AlF the solvent, at a temperature of about 970 C. The dissolved alumina, when subjected to a high intensity current, in electrolytic cells of either the continuous, self-baking Soderberg anode type or the pre-baked anode type, disassociates into positive aluminum and negative oxygen ions. In practice, a plurality of substantially identical electrolytic reduction cells, for example 28 reduction cells, are arranged in a pot line, that is, they are connected electrically in series. A direct current of from about 50,000 amperes to 160,000 amperes or more, in commercial reduction systems, is usual. The electrical path for the external current source is composed of the carbon anode structure, the electrolytic bath and the cathode structure, usually in the form of collector bars buried in the bottom of the reduction cell. The specific current, in any case being determined by the size of the electrolytic reduction cells, flows through the bath containing the alumina and electrolyte, a voltage drop of from about fourvolts to about 6 volts appearing across each reduction cell during normal electrolysis. As the normal electrolysis preceeds, aluminum is deposited at the cathodic bottom of the reduction cell or each of the series-connected reduction cells where it collects as a molten pool of aluminum, a tap being provided in each of the reduction cells so that the aluminum can be periodically removed. The side of the reduction cell which is provided with the tap for removing the molten aluminum is known as the tap side. The oxygen of the alumina combines with the carbon of the anode to form principally carbon dioxide and carbon monoxide, the gases being conventionally led away from each reduction cell by a duct, the duct being positioned near the top of that side of each cell which is opposite to the tap side. This particular side of a reduction cell is referred to generally as the duct side.

According to Faradays Law, the pounds of aluminum produced are directly proportional to the quantity of electrical charge passed through each of the reduction cells. An approximate equivalent circuit for an individual reduction cell would show a decomposition voltage of back EMF in series with a resistance having a fixed component and variable component. The fixed component is determined by the electrical resistance of mechanical circuit connections, while the variable component represents the resistance of the bath itself. The bath resistance, in turn, can be expressed as R,

pD/A, where A, the effective surface area, is essentially constant, but Rho, the resistivity, varies with the alumina concentration, and D, the anode to cathode spacing, varies with anode consumption as well as with aluminum buildup on the cathodic bottom of the reduction cell. The thermal input to the reduction cell, and hence temperature of the cell, depends on the R losses generated in this resistance.

The efficiency of the process, at least in terms of pounds per ampere-hour is determined by the percentage of metallic aluminum which, under the influence of strong magnetic fields, comes into contact with oxygen near the anode to reform the original oxide. The tendency for this to happen increases with temperature and decreases with the distance between the anode and the cathode. Although aluminum production depends on ampere-hours, users of electrical power actually pay on a kilowatt-hour basis, consequently, efficiency in terms of cost depends on maximizing current in the volt-ampere relationship. For most efficient operation, the resistance of the reduction cell should be regulated to provide a low stable bath temperature with high current and minimum reoxidation.

One of the continuing problems encountered in the commercial electrolytic aluminum reduction process is the effective control of the concentration of dissolved alumina in the bath. If the concentration of alumina is depleted from the upper maximum of from about 7 percent to about 10 percent down to a certain critical limit, generally considered to be approximately 2.0 percent, a phenomena known as anode effect occurs, with its consequent well-known disadvantages and reduced efficiency. The anode effect is a characteristic of reduction cells in which aluminum is being produced by electrolysis of a cryolite/alumina bath. The anode effect is conventionally extinguished and normal electrolysis restored by the expedient of breaking the frozen top crust of the bath which adds alumina into the bath. Extreme caution must be taken, however, not to charge the bath with too much additional alumina for all of the additional alumina will not dissolve if the amount exceeds a solubility capacity of the electrolyte for alumina at the prevailing temperature, usually about 970 C. If the electrolyte cannot dissolve all of the additionally added alumina, some of the alumina will sink through the electrolyte and through the molten alumina, collecting on the cathodic bottom surface of the reduction cell, with the result that the resistance of the cathode undesirably increases, efficiency declines resulting in what is known as an over-fed or sick reduction cell.

In both cases the anode effect, which results from a starvationcondition of the bath, and the sick cell phenomena which results from overfeeding the bath, the reduction cell is working under abnormal conditions with the concomitant undesirable decline in overall efficiency. Of the two conditions, the anode effect has been found to be the lesser of the two disadvantages for it can be extinguished more easily than the sick cell condition can be remedied. Consequently, techniques have been developed, involving both intermittent and continuous alumina feeding of an electrolytic bath, which add alumina to the electrolytic bath routinely in amounts adapted to avoid development of a sick cell condition. Such feeding techniques rely on an underfeeding practice, which allows the reduction cell to undergo occasional anode effects, for example, one anode effect per day, which assures against overfeeding alumina into the reduction cell.

The U.S. Pat. No. 3,400,062, to Bruno et al, issued Sept. 3, 1968, discloses a control system for an aluminum reduction cell having an anode, a cathode and a fused electrolytic bath of cryolite and dissolved alumina. A pilot anode is insulatively supported at the reduction cell, with one end extending into the electrolytic bath to a given immersion depth. A power supply is provided for supplying direct current energy to the pilot anode. The current density supplied to the pilot anode during a standby, which may be of several minutes duration, is sufficiently high to maintain the auxiliary anode on anode effect. This particular condition, according to the patent specification, is maintained to prevent consumption of the pilot anode by the electrolysis. During a second period of from about 8 to about 10 seconds duration, for example, the current to the pilot anode is reversed in order to eliminate its anode effect. Finally, during a third period of from about 10 to about seconds a second or lower voltage, preselected in dependence on the level of alumina controlled desired, is impressed upon the pilot anode causing the resulting current through it in the forward direction to provide a given control current through it in the forward direction to provide a given control current density in order to determine the alumina concentration of the electrolytic bath by sensing whether or not an anode effect appears on the pilot anode under this lower voltage condition. After a brief pause of a few seconds to allow the current in the pilot anode to stabilize, a sensor associated with the pilot anode is made effective for detecting whether or not an anode effect has appeared. If an anode effect has appeared, it is an indication that the electrolytic bath is in an underfed condition, and the feed rate for the alumina into the bath is increased. On the other hand, if no anode effect appears during this sensing period, the normal slower feed rate is maintained. Thus, it is clear that the system disclosed in the prior patent to Bruno et al is rather complex and, so far as the reverse current and sensing periods are concerned, requires from about 18 to about 38 seconds to operate.

The U.S. Pat. No. 3,539,456, to Smids, issued Nov. 10, 1970, discloses an electrolytic cell solute determining apparatus for use in the operation of a direct current electrolytic reduction call in which a pair of alternating current energized auxiliary electrodes, which extend into an aluminum oxide/solute containing bath of the electrolytic reduction cell, serves as a means for sensing the current of cycling anode effects induced thereon during operation of the reduction cell. This particular apparatus, because the alternating current energization of the pilot electrodes, does not require the current reversal in the pilot electrode as was needed in the direct current energized pilot electrode arrangement disclosed in the above-mentioned patent to Bruno et al. It is to be appreciated, however, that the anode effect sensing apparatus disclosed in the abovementioned patent to Smids still requires a relatively long period to operate and requires that a special alternating current supply and auxiliary electrodes be provided.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method of producing metal from an electrolytic bath which involves sensing the occurrence of a voltage level in excess of a given level across the bath for determining the existence of anode effects.

It is another object of the present invention to provide a method of producing metal from an electrolytic bath which involves sensing the occurrence of a voltage level in excess of a given level across the bath for determining the existence of anode effects and the extinguishing of such effects.

It is a further object of the present invention to provide, in an apparatus for the reduction of metal from an electrolytic bath, a fast-acting circuit arrangement for the detection of anode effects.

It is yet another object of the present invention to provide, in an apparatus for the reduction of metal from an electrolytic bath, a circuit arrangement which senses the occurrence of an anode effect and effects the extinguishing of the detected anode effects.

It is yet a further object of the present invention to provide, in an apparatus for the reduction of metal from an electrolytic bath, an inexpensive and reliable circuit arrangement for sensing the occurrence of an anode effect. 7

It is still another object of the present invention to provide, in an apparatus for the production of aluminum from an electrolytic cell, a circuit arrangement which does not involve the use of auxliary or pilot electrodes.

The foregoing objects, as well as others which are to be made apparent from the text below, are accomplished according to the present invention in its method aspect by providing an electrolytic bath containing dis.- solved oxide of the metal to be produced in a reduction cell, causing direct current to flow through the bath and collecting the metal produced on the bottom of the reduction cell. The method includes sensing the direct voltage drop across the cell and determining when this voltage drop exceeds a given level as an indication of anode effect. The method may also involve extinguishing the anode effect.

The foregoing objects, as well as others which are to be made apparent from the text below, are accomplished according to the present invention in its apparatus aspect by providing an anode effect detector in an apparatus for producing metal from an electrolytic bath containing dissolved oxide of the metal. The apparatus includes at least one reduction cell having electrode means for delivering direct current to the bath. The anode effect detector is operatively arranged to sense the voltage of the reduction cell and provides an output signal whenever the voltage of the reduction cell exceeds a given level indicating the existence of an anode effect. The apparatus further may include devices responsive to the output from the anode effect detector which extinguish the detected anode effect.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE is a schematic illustration of an apparatus for producing metal from an electrolytic bath in accordance with an illustrative apparatus embodiment of the present invention, the apparatus being particularly suitable for carrying out the method according to the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT I An alumina reduction cell, generally designated 9,

S with associated circuitry, suitable for practising the present invention is shown schematically. The alumina reduction cell 9 includes a steel shell 10 having a carbonaceous lining 11. The conductive lining 11 contains a pool of molten aluminum 12 and a bath 13 of alumina dissolved in a molten electrolyte, the bath 13 being above the pool of molten aluminum l2. Conductive rods, which are embedded in the conductive lining 13, are connected to a cathode conductor or bus 14. It is to be understood that other forms of lining can be used to contain the molten aluminum 12 and the bath 13. A cathode potential can be impressed on the molten aluminum 12 by other conventional means instead of the conductive rods as shown. Suspended above the bath 13, and partially immersed therein, is a carbon anode 15 shown diagrammatically. In practice, the carbon anode 15 may be a multiple bar anode arrangement positioned on a suitable superstructure adjustable as a unit or a conventional vertical or horizontal stub Soderberg-type anode. One multiple bar anode arrangement which can be used for the anode 15 comprises 18 carbon bars, each weighing about 1 ton. The molten bath 13 is covered by a hard crust 16 which consists of frozen electrolyte constituents and additional alumina. The anode 15 is connected to a positive bus 17 via a conductor 18. A current sensing device 20 is provided for sensing the current flowing in the conductor 18. The current sensing device 20, which produces a direct voltage directly related to the direct current flowing in the conductor 18, preferably is of a type which does not require a series connection in the conductor 18.

On the tap side of the reduction cell 9, a first conventional alumina feeder 24 is provided. A first crust breaking bar 25 is provided in the vicinity of the first feeder 24. A second alumina feeder 26 is provided on the duct side of the reduction cell 9 and a second crust breaking bar 27 is provided in the vicinity of the second feeder 26. Two pneumatically or electrically operated motion-producing devices 28 and 30 mechanically connected to the anode 15 are provided respectively for raising and lowering the anode 15 in predetermined increments. A volt meter 31 is connected between the negative bus 14 and the conductor 18.

A pulse producing timing circuit 32 is provided for producing two pulse trains, each having an identical pulse repetition rate, for example, a pulse repetition rate of 6 pulses per minute. The two pulse trains are out of phase, one pulse train being displaced from the other by one-half the interval between pulses, for example, by 5 seconds. One of the pulse trains from the timing circuit 32 is fed to the enabling input of a gate circuit 33 and the other pulse train is fed to the enabling input of a gate circuit 34. The signal input of the gate circuit 33 is connected to the current sensing device 20 and receives therefrom a voltage signal directly propor tional to the current flowing in the conductor 18. The signal input of the gate circuit 34 is connected to the conductor 18 and receives therefrom a voltage corresponding to the voltage across the reduction cell 9.

The respective outputs from the gate circuit 33 and the gate circuit 34 are coupled to the input of a limiting amplifier 35 which is preferably operatively arranged to limit at an input voltage of approximately ten volts. The limiting amplifier 35 preferably has a gain of one. The output from the limiting amplifier 35 is coupled to an analog to digital converter 36 which produces binary coded digital signal outputs which correspond, at

different times, to the current supplied to the reduction cell 9 and to the voltage drop across the reduction cell 9, as determined by which of the two gate circuits 33 and 34 is supplying an input to the limiting amplifier 35.

The output from the analog to digital converter 36 is coupled to the first input of an AND circuit 37 and to the first input of an AND circuit 38, second inputs to the AND circuit 37 and to the AND circuit 38 being connected to the timing circuit 32 for receiving the respective pulse trains therefrom. Thus, the AND circuit 37 intermittently passes to its output a binary coded digital signal indicative of the direct current flowing within the reduction cell 9 and the AND circuit 38 intermittently passes into its output a binary coded digital signal indicative of the voltage across the reduction cell 9.

The AND circuit 38 has its output coupled to a first input of a subtractor 39. A second input to the subtractor 39 is connected to a binary coded digital signal source 29 which is settable and provides as its signal output a predetermined binary coded digital signal representing the back EMF of the reduction cell 9, this back EMF being nominally 1.6 volts for an alumina/- molten cryolite bath. The output signal from the subtractor 39 is fed to a first input of an arithmetical circuit, denominated as an arithmatic divider 40. The AND circuit 37 has its output coupled to a second input of the divider 40 via a digital store 41 which stores the binary coded digital signal received from the AND circuit 37 for a sufficiently long period to assure that the divider 40 has present contemporaneously at its two inputs the signals passed by the AND circuits 37 and 38. The divider 40 produces as its output a binary coded digital signal which is the quotient of the digital signal representing the gross voltage across the reduction cell being examined minus the back EMF of the cell divided by the digital signal representing current, its binary coded digital output signal thus corresponds to the resistance of the reduction cell 9, its electrodes and connections thereto.

The output from the divider 40 is coupled to a first input of an arithmatic subtractor 43 which is operatively arranged to receive at its second input a predetermined binary coded digital signal from a digital signal source 42, which signal represents the known fixed electrical resistance of the electrical connections to the reduction cell 9. Accordingly, the subtractor 43 produces as its output signal a binary coded signal substantially directly corresponding to the varying resistance of the bath 13.

The output from the subtractor 43 is coupled to first inputs of a first digital signal comparator 44 and a second digital signal comparator 45. A second input to the digital signal comparator 44 is provided from an upper threshold setting circuit 46 which is a source of a binary coded digital signal for establishing an upper resistance value for the bath 13, the alumina concentration being directly related to the resistance of the bath 13. A second input to the second digital comparator 45 is provided from a lower threshold setting circuit 47. The comparator 44 provides an output whenever the digital signal it receives from the subtractor 43 exceeds the digital signal it receives from the upper threshold setting circuit 46, indicating that the resistance of the bath 13 is too high. The output from the comparator 45 appears whenever the digital signal it receives from the subtractor 43 is less than the digital signal it receives from the lower threshold setting circuit 47, indicating that the resistance of the bath 13 is too low. It is to be understood that the digital signal source 42 and the subtractor 43 are not necessary, the output from the divider 40 could be directly coupled to the comparators 44 and 45 provided that the threshold setting circuits were appropriately set to include the fixed resistance of the electrical connections to the reduction cell 9.

The output from the limiting amplifier 35 is also connected to an anode effect detector 48 which is a Zener diode having a voltage switching threshold of approximately 7.5 volts. Since the anode effect detector 48 has a voltage threshold of 7.5 volts, it will not conduct and will not produce an output signal so long as the voltage of the reduction cell 9 remains within the range below 7.5 volts, the expected range being from about 3.5 volts to about 6.5 volts, 5.0 volts rarely being exceeded, during normal bath conditions. Whenever the voltage across the reduction cell 9 increases above the 7.5 volt level, the anode effect detector 48 conducts producing a logical ONE signal on its output, indicating that the reduction cell 9 is undergoing an anode effect which signals that the concentration of alumina in the bath 13 is much too low for efficient operation. Since anode effect may and often does produce voltages as high as 30 or 40 volts across a reduction cell, the limiting amplifier 35 is arranged to limit at an input of at about volts thereby preventing damage to the analog to digital converter 36 and to the anode effect detector 48 Without decreasing the sensitivity of the circuitry.

As discussed above, the circuitry as thus far described in effect determines the resistance of the reduction cell six times every minute. In practical applications of the present invention, the resistance of the reduction cell may be determined at greater intervals, for example, at one minute intervals.

Three digital signal sources 50, 51 and 52 are provided. Each of the digital signal sources 50, 51 and 52 include respective stores 53, 54 and 55 which respectively store a regular normal break and feed program, a resistance control, anode position adjusting program and an anode effect extinguishing program. The stored programs, in each instance, are respective stored binary coded digital signals in bit parallel and command serial.

The digital signal source 50 provides in command sequence and bit parallel a series of binary coded digital command signals from its store 53 to effect, in sequence, the breaking of the crust 16 on the tap side by the breaker bar 25, the feeding of additional alumina to the tap side from the feeder 24, the breaking of the crust 16 on the duct side by the breaker bar 27 and the feeding of additional alumina to the duct side from the feeder 26. The breaking bars 25 and 27 are, in most practical instances, moved up and down several times to assure that the crust 16 is broken, the digital signal source 50 from its store 53 supplying the appropriate command signal or signals for effecting such multiple motions.

In a practical instance, the digital signal source 50 supplies the digital command signals, in bit parallel, which effects first a breaking at the tap side, with subsequent feeding of the tap side at a predetermined later time and thereafter, usually approximately 90 minutes later, the breaking and subsequent feeding of the duct side of the reduction cell 9. Since the crust 16 is predominantly alumina, the breaking of the crust 16 enriches the bath 13, resulting in a lowering of the bath resistance. The feeding may also provide, if desired, additional alumina to the bath i3, but is preferably done at a time sufficiently later than the breaking so that the newly fed alumina becomes part of the crust 16 or is supported on its surface. The digital signals, in bit parallel, are supplied from the output of the digital signal source to a command decoder via a series connected negated AND circuit 56, a negated AND circuit 57 and an OR circuit 58.

The digital signal source 51 is provided with two en abling inputs which are supplied respectively from the comparator 44 and the comparator 45. In response to a digital difference signal from the comparator 44, indicating that the upper threshold set point for the resistance of the electrolytic reduction cell 9 has been exceeded, the digital signal source 51 is operatively arranged to supply from its store 54 a binary coded digital signal, in bit parallel, to the command decoder 55 calling for the anode 15 to be lowered by a given increment or increments depending on the magnitude of the digital difference signal supplied from the comparator 44. Thus, the resistance of the reduction cell 9 is lowered until the digital difference signal from the comparator 44 disappears. In response to a digital difference signal from the comparator 45, indicating that the lower threshold set point for the resistance of the reduction cell 9 has been exceeded, the digital signal source 51 is operatively arranged to supply from its store 54 a binary coded digital signal, in bit parallel, to the command decoder 55 calling for the anode 15 to be raised by a given increment or increments depending on the magnitude of the digital difference signal supplied from the comparator 45. Consequently, the resistance of the reduction cell 9 is increased until the digital difference signal from the comparator 45 disappears.

The binary coded digital signals from the digital signal source 51 which call for either an incremental lowering or an incremental raising of the anode 15, are supplied to the command decoder 55 via a negated AND circuit and the OR circuit 58. A second output from the digital signal source 51, which simply indicates that the digital signal source 51 is supplying signals to effect anode movement, is coupled to the negated input of the AND circuit 57 thereby interrupting the regular break and feed program fed to the command decoder 55 from the digital signal source 50.

The digital signal source 51 as thus far described responds whenever difference signals appear on either the output from the comparator 44 or the comparator 45. The digital signal source 51 is preferably so constructed that it inhibits itself from supplying command signals for a period of 5 minutes after each of its responses.

The output from the anode effect detector 48, which appears as a logical ONE whenever its input exceeds 7.5 volts by virtue of the Zener characteristic of the detector 48, is coupled to the enabling input of the digital signal source 52. Whenever the digital signal source 52 is enabled, it produces from its store 54 a series of binary coded digital command signals, in bit parallel, to effect in sequence the breaking of the crust 36 on both the tap side and the duct side of the reduction cell 9, the lowering of the anode i5, and the subsequent feeding of the reduction cell from both the feeder 24 and the feeder 26. As in the normal breaking and feeding operation, the feeding operations preferably take place during an anode effect extinguishing operation after the crust 16 has hardened. In some instances, it may be sufficient to break and feed only either the duct side or the tap side to assure anode effect suppression.

The output digital command signals, in bit parallel, are coupled to the command decoder 55 via the OR circuit 58.

A second output from the digital signal source 52, which simply indicates that the digital signal source 52 is providing an anode effect extinguishing command signals, is coupled to the negated inputs of the AND circuit 60 and of the AND circuit 56 for the purpose of disabling feed of the regular break and feed program routine signals to the command decoder 55.

Thus, the digital signal sources 50, 51 and 52 supply to the exclusion of each other and on a priority basis binary coded digital command signals, in bit parallel, to the command decoder 55 which, in turn, produces 6 output signals on its output lines 61-66 which are fed to respective memory circuits 67-72. The memory circuits 67-72 in turn supply signals to respective alternating current solenoid drivers 73-78. The memory circuits 67-72, which may be in the form of long RC time constant circuits, are provided to assure that the output of the command decoder 55 is present sufficiently long to energize their associated respective solenoid drivers 73-78, and at the same time freeing the command decoder 55 for the decoding of additional command signals.

The solenoid drivers 73 and 78, which respond respectively to signals stored in the memory circuit 67 and in the memory circuit 72, are arranged to energize respectively the first feeder 24 on the tap side and the second feeder 26 on the duct side of the electrolytic cell 9. The feeders 24 and 27 are of conventional construction and preferably are operated by pneumatically or electrically responsive devices respectively controlled from the solenoid driver 73 and the solenoid driver 78. The solenoid drivers 74 and 77, which respond respectively to signals stored in the memory circuit 68 and in the memory circuit 71, are arranged to energize respectively first and second pneumatically or electrically operated devices 80 and 81 which are mechanically coupled respective to the breaker bars 25 and 27 to effect movement of them.

The solenoid drivers 75 and 76, which respond respectively to signals stored in the memory circuit 69 and the memory circuit 70, are arranged to energize respectively the pneumatically or electrically operated motion-producing device 30 and the pneumatically or electrically operated motion-producing device 28 which are respectively operatively arranged to effect the lowering and the raising of the anode 15.

In order to place the apparatus of the present invention in a condition ready for operation, suitable programs in the form of binary coded digital signals for the regular, normal breaking and feeding function, for the resistance control function and for the anode effect extinguishing function are placed respectively in the stores 53, 54 and 55. Having determined, by conventional techniques, the substantially fixed electrical resistance of the electrical connections to the reduction cell 9, the digital signal source 42 is set to provide, as its output signal, a binary coded digital signal representative of such resistance. The digital signal source 29 is set to provide, as its output signal, a binary coded digital signal representative of the predetermined back EMF of the reduction cell 9, this back EMF being for a suitable alumina/cryolite bath 1.6 volts.

The upper threshold setting circuit 46 is set to provide, as its output signal, a fixed binary coded digital signal which corresponds to the upper limit (i.e., 20.1 X 10 ohms) of the resistance range for the electrolytic bath 13 during expected normal electrolysis. This set point, for example, corresponds closely to that point at which the gross voltage across the reduction'cell 9 would have increased by substantially +0.02 volts at a nominal current of 150,000 amperes. The lower threshold setting circuit 47 is set to provide, as its output signal, a fixed binary coded digital signal which corresponds to the lower limit (i.e., 19.9 X 10 ohms) of the resistance range for the electrolytic bath 13 during expected normal electrolysis. This set point, for example, corresponds closely to that point at which the gross voltage across the reduction cell 9 would have decreased by substantially 0.02 volts at the nominal current of 150,000 amperes. It is to be understood that difference set points could be used if desired, as determined by desired bath conditions and the sensitivity of the control desired in an given case.

The reduction cell 9 is charged with the appropriate amount of solvent, NaF/AlF and alumina, A1 0 which charge forms the electrolytic bath. The reduction process is initiated preferably manually by supplying direct current to the reduction cell 9, with the possible addition of heat from auxiliary heating means, and adjusting manually the position of the anode 15, with respect to the cathode bottom of the reduction cell until the voltage across the reduction cell 9, as readable from the voltmeter 31, and the direct current to the reduction cell 9, as determined by the current sensing device 20, are within limits known to provide efficient operation.

Once normal electrolysis is progressing, the digital signal source 50 is brought into operation supplying regular break and feed digital command signals to the command decoder 55 which responds to such signals by sequentially signaling, via the memory circuits 68, 67, 71 and 72, the solenoid drivers 74, 73, 77 and 78 whichh, in turn effect the movement of the breaking bar 25, the feeder 24, the breaking bar 27 and the feeder 26. In normal operation, the tap side of the reduction cell 9 is thus broken and fed every 180 minutes, a delay period being provided between the breaking and feeding. The duct side of the electrolytic cell 9 is thus broken and fed also every 180 minutes, the times of each being displaced by minutes from the corresponding breaking and feeding at the tap side of the reduction cell.

Electrolysis continues, the circuitry automatically determining the resistance of the bath 13, appropriate signals being produced by the comparator 44 and the comparator 45 which, whenever the resistance of the bath 13 becomes either too high or too low, signal the digital signal source 51 which supplies digital command signals to the decoder 55. The decoder 55 responds by producing, as the case may be, an output signal to either the memory circuit 69 or tthe memory circuit 70, which cause the anode 15 to be either raised or lowered. This is accomplished by the motion-producing devices 28 and 30 controlled from the solenoid drivers 75 and '76, which respond to the signals stored in the memory circuits 70 and 69 respectively. Whenever the digital signal source 51 is supplying output signals, the output from the digital signal source 50 is effectively prevented from reaching the command decoder 55 because of the fact that a signal from the digital signal source 51 is coupled to the negated input of the AND circuit 57.

During operation, the voltage across the reduction cell 9 is intermittently sensed, by action of the gate circuit 34, the voltage signal being passed by the limiting amplifier 36, which has a gain of one, its output in turn being supplied to the anode effect detector 48 which conducts whenever the voltage exceeds 7.5 volts, its Zener breakdown voltage. The anode effect detector 48 responds within a few microseconds, much faster than the to 50 millisecond response time of analog to digital converter 36, supplying a logical ONE signal to the digital signal source 52 which produces a series of digital command signals to the command decoder 55 to cause, in succession, the crust 16 on the bath 13 to be broken, possibly on both the tap side and the duct side of the reduction cell 9, the anode 15 to be lowered, and subsequent feeding of one or both sides of the reduction cell. The mechanical movements are effected by the solenoid drivers 75, 74, 73, 77 and 78. The digital signal 52 source also preferably produces a digital command signal which is decoded by the decoder 55 and fed to the solenoid driver 76, via the memory circuit 70, to causethe anode 15 to be returned to its earlier position.

A separate output from the digital signal source 52 is fed to the negated inputs of the AND circuits 56 and 60 to assure that no command signals from the digital signal sources 50 and 51 are supplied to the command decoder 55 when it is receiving command signals from the digital signal source 52.

The method of producing metal according to the present invention, in its broadest aspect, involves the steps of providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct current to flow through the bath, collecting the metal produced on the bottom of the reduction cell, sensing the direct voltage across the reduction cell and determining when such voltage exceeds a given level as an indication of anode effect.

In a further aspect, the method according to the present invention includeswthe step of extinguishing the anode effect or effects which may occur.

The method according to the present invention preferably includes the routine feeding of the oxide of the metal into the bath at a rate insufficient to maintain the concentration of such material in the bath at levels which avoid the occurrence of anode effects. This avoids the more serious problems which would result from overfeeding the reduction cell, causing a sick cell condition.

The method according to a preferred aspect, involves the production of aluminum. In this case the electrolytic bath is composed of alumina as the solute and cryolite as the solvent.

The step of sensing the direct voltage across the reduction cell is preferably done intermittently.

In another preferred aspect, the method according to the present invention involves the steps of determining the resistance of the bath and adjusting the anode-tocathode electrode spacing to maintain the resistance of the bath within predetermined limits during regular electrolysis. t

The step of extinguishing the anode effects fwhich may occur includes in preferred aspects of the present invention reducing the anode-to-cathode spacing of the electrode structure of the reduction cell and breaking crust present on the surface of the bath. Additional metal oxide of the metal to be produced may be fed into the reduction cell subsequent to the breaking of the crust.

Although the present invention has been described, in its apparatus aspect, in conjunction with a single electrolytic reduction cell, it is to be appreciated that the invention is applicable to systems which involve multiplexing of the command signals in order to control the operating parameters of many reduction cells. In this instance, the circuit arrangement would, of course, also sense the currents supplied to each cell, the voltage across each cell via multiplexing circuits, the feeding of the command signals and the sensing of the currents and the voltages being appropriately synchronized.

It is also to be appreciated that the invention, in its method aspect, need not be carried out in the illustrated apparatus, but may be carried out by other apparatuses.

While one embodiment of the invention has been shown for purposes of illustration, it is to be understood that various changes in the details of construction and arrangement of parts may be made without departing from the spirit and scope of the invention as defined in the appended method and apparatus claims.

It is claimed:

1. A method of producing metal comprising providing an electrolytic bath containing dissolved oxide of the metal in a reduction cell, causing direct currentto flow through said bath, collecting said metal on the bottom of said reduction cell, sensing the direct voltage across said cell, producing a limited output signal corresponding to the voltage across said cell up to a given level thereof, which is less than maximum anode effect voltage level, and producing a signal of ONE level upon occurrence of any output signal level if the limited output signal is in excess of a given threshold level, which is less than maximum possible level of the limited output signal, and developing a signal of ZERO level upon occurrence of any output signal level below the given threshold level, the signal of ONE level being indicative of occurence of anode effect.

2. A method as defined in claim 1 further comprising extinguishing the anode effect.

3. A method as defined in claim 1 wherein said step of providing an electrolytic bath includes routinely feeding the oxide of the metal into the bath at a rate insufficient to maintain the concentration of such material in said bath at levels which avoid the occurrence of anode effects.

4. A method as defined in claim 2 wherein said step of providing an electrolytic bath includes routinely feeding the oxide of the metal into the bath at a rate insufficient to maintain the concentration of such material in said bath at levels which avoid the occurrence of anode effects.

5.'A method as defined in claim 1 wherein the step of providing an electrolytic bath comprises providing an electrolytic bath composed of alumina as the solute l3 and cryolite as the solvent. the metal produced being aluminum.

6. A method as defined in claim ll wherein the step of sensing the direct voltage across said reduction cell is the step of intermittently sensing the voltage across said reduction cell.

'7. A method as defined in claim 1 further comprising determining the resistance of said bath, and adjusting the anode-to-cathode electrode spacing of the electrode structure of said reduction cell to maintain the resistance of said bath within predetermined limits during regular electrolysis.

8. A method defined in claim 2 wherein the step of extinguishing the anode effect comprises reducing the anode-to-cathode spacing of the electrode structure of said reduction cell and breaking crust present on the surface of said bath.

9. A method as defined in claim 8 further including, subsequent to the crust breaking, feeding additional metal oxide of the metal to be produced into said reduction cell.

10. An apparatus for producing metal from an elec trolytic bath containing dissolved oxide of the metal comprising at least one reduction cell having electrode means for delivering direct current to said electrolytic bath, said electrode means including anode electrode means and cathode electrode means; and anode effect detecting means including voltage limiting means responsive to voltage between said anode electrode means and said cathode electrode means for developing an output signal corresponding to the voltage between said anode eiectrodc means and said cathode electrode means only up to a given level thereof below maximum anode effect voltage between these electrode means, and threshold level detector means responsive to the output signal from said limiting means for developing a signal of ONE level in response to signals exceeding a given threshold level which is less than maximum output signal level of said limiting means and for developing a signal of ZERO level in response to signals below the given threshold level from said limiting means, the signal of the ONE level being indicative of occurrence of an anode effect.

11. An apparatus as defined in claim it) wherein said threshold level detecting means is a diode which becomes conductive at approximately 7.5 volts, said diode being substantially nonconductive at lower voltage levels.

12. An apparatus as defined in claim 11 wherein said limiting means is a limiting amplifier, said amplifier being operatively arranged to limit at approximately 10.0 volts and having a gain of one.

13. An apparatus as defined in claim 10 further com prising means responsive to the output of said threshold level detecting means for extinguishing the detected anode effects.

14. An apparatus as defined in claim 13 wherein said means for extinguishing the detected anode effects includes means for reducing the spacing between said anode electrode means and said cathode electrode means and means for breaking crust on the surface of said bath.

15. An apparatus as defined in claim 14 wherein said means for extinguishing the detected anode effects includes means for feeding additional oxide of the metal to be produced into said reduction cell.

16. An apparatus as defined. in claim 15 further comprising means for routinely feeding oxide of the metal into said bath.

17. An apparatus as defined in claim to wherein said means for routinely feeding oxide of the metal into said bath is operatively arranged to feed such material at a rate insufficient to avoid all anode effects.

18. An apparatus as defined in claim 10 further comprising means for determining the resistance of said bath and means responsive to output from said means for determining resistance for adjusting spacing between said anode electrode means and said cathode electrode means to maintain the resistance of said bath within predetermined limits.

19. An apparatus as defined in claim 13 wherein said means for extinguishing the detected anode effects includes means for reducing spacing between said anode electrode means and said cathode electrode means in response to signals from said threshold level detecting means.

20. An apparatus as defined in claim 19 wherein said means for extinguishing the detected anode effects further includes means for feeding additional oxide of the metal to be produced into said reduction cell in response to signals from said anode effect detecting means. 

1. A METHOD OF PRODUCING METAL COMPRISING PROVIDING AN ELECTROLYTIC BATH CONTAINING DISSOLVED OXIDE OF THE METAL IN A REDUCTION CELL, CAUSING DIRECT CURRENT TO FLOW THROUGH SAID BATH, COLLECTING SAID METAL ON THE BOTTOM OF SAID REDUCTION CELL, SENSING THE DIRECT VOLTAGE ACROSS SAID CELL, PRODUCING A LIMITED OUTPUT SIGNAL CORRESPONDING TO THE VOLTAGE ACROSS SAID CELL UP TO A GIVEN LEVEL THEREOF, WHICH IS LESS THAN MAXIMYMUM ANODE EFFECT VOLTAGE LEVEL, AND PRODUCING A SIGNAL OF ONE LEVEL UPON OCCURRENCE OF ANY OUTPUT SIGNAL LEVEL IF THE LIMITED OUTPUT SIGNAL IS IN EXCESS OF A GIVEN THRESHOLD LEVEL, WHICH IS LESS THAN MAXIMUM POSSIBLE LEVEL OF THE LIMITED OUTPUT SIGNAL, AND DEVELOPING A SIGNAL OF ZERO LEVEL UPON OCCURRENCE OF ANY OUTPUT SIGNAL LEVEL BELOW THE GIVEN THRESHOLD LEVEL, THE SIGNAL OF ONE LEVEL BEING INDICATIVE OF OCCURRENCE OF ANODE EFFECT.
 2. A method as defined in claim 1 further comprising extinguishing the anode effect.
 3. A method as defined in claim 1 wherein said step of providing an electrolytic bath includes routinely feeding the oxide of the metal into the bath at a rate insufficient to maintain the concentration of such material in said bath at levels which avoid the occurrence of anode effects.
 4. A method as defined in claim 2 wherein said step of providing an electrolytic bath includes routinely feeding the oxide of the metal into the bath at a rate insufficient to maintain the concentration of such material in said bath at levels which avoid the occurrence of anode effects.
 5. A method as defined in claim 1 wherein the step of providing an electrolytic bath comprises providing an electrolytic bath composed of alumina as the solute and cryolite as the solvent, the metal produced being aluminum.
 6. A method as defined in claim 1 whereIn the step of sensing the direct voltage across said reduction cell is the step of intermittently sensing the voltage across said reduction cell.
 7. A method as defined in claim 1 further comprising determining the resistance of said bath, and adjusting the anode-to-cathode electrode spacing of the electrode structure of said reduction cell to maintain the resistance of said bath within predetermined limits during regular electrolysis.
 8. A method as defined in claim 2 wherein the step of extinguishing the anode effect comprises reducing the anode-to-cathode spacing of the electrode structure of said reduction cell and breaking crust present on the surface of said bath.
 9. A method as defined in claim 8 further including, subsequent to the crust breaking, feeding additional metal oxide of the metal to be produced into said reduction cell.
 10. An apparatus for producing metal from an electrolytic bath containing dissolved oxide of the metal comprising at least one reduction cell having electrode means for delivering direct current to said electrolytic bath, said electrode means including anode electrode means and cathode electrode means; and anode effect detecting means including voltage limiting means responsive to voltage between said anode electrode means and said cathode electrode means for developing an output signal corresponding to the voltage between said anode electrode means and said cathode electrode means only up to a given level thereof below maximum anode effect voltage between these electrode means, and threshold level detector means responsive to the output signal from said limiting means for developing a signal of ONE level in response to signals exceeding a given threshold level which is less than maximum output signal level of said limiting means and for developing a signal of ZERO level in response to signals below the given threshold level from said limiting means, the signal of the ONE level being indicative of occurrence of an anode effect.
 11. An apparatus as defined in claim 10 wherein said threshold level detecting means is a diode which becomes conductive at approximately 7.5 volts, said diode being substantially nonconductive at lower voltage levels.
 12. An apparatus as defined in claim 11 wherein said limiting means is a limiting amplifier, said amplifier being operatively arranged to limit at approximately 10.0 volts and having a gain of one.
 13. An apparatus as defined in claim 10 further comprising means responsive to the output of said threshold level detecting means for extinguishing the detected anode effects.
 14. An apparatus as defined in claim 13 wherein said means for extinguishing the detected anode effects includes means for reducing the spacing between said anode electrode means and said cathode electrode means and means for breaking crust on the surface of said bath.
 15. An apparatus as defined in claim 14 wherein said means for extinguishing the detected anode effects includes means for feeding additional oxide of the metal to be produced into said reduction cell.
 16. An apparatus as defined in claim 15 further comprising means for routinely feeding oxide of the metal into said bath.
 17. An apparatus as defined in claim 16 wherein said means for routinely feeding oxide of the metal into said bath is operatively arranged to feed such material at a rate insufficient to avoid all anode effects.
 18. An apparatus as defined in claim 10 further comprising means for determining the resistance of said bath and means responsive to output from said means for determining resistance for adjusting spacing between said anode electrode means and said cathode electrode means to maintain the resistance of said bath within predetermined limits.
 19. An apparatus as defined in claim 13 wherein said means for extinguishing the detected anode effects includes means for reducing spacing between said anode electrode means and said cathode electrode means in response to signals fRom said threshold level detecting means.
 20. An apparatus as defined in claim 19 wherein said means for extinguishing the detected anode effects further includes means for feeding additional oxide of the metal to be produced into said reduction cell in response to signals from said anode effect detecting means. 